Terrestrial mobile telecommunications systems are well known and a number of different systems have developed which operate according to different standards, both analog and digital. In Europe and the Far East, excluding Japan, and elsewhere, the digital Global System Mobile (GSM) network has become popular, whereas in the USA, networks which operate according to the IS-41 recommendations such as the Advanced Mobile Phone System (AMPS) and the Digital Advanced Mobile Phone System (DAMPS) are used. In Japan, the Personal Handiphone System (PHS) and the Personal Digital Communication (PDC) network are in use. More recently, proposals have been made for a Universal Mobile Telecommunications System (UMTS). These networks are all cellular and land-based but have differences in architecture and use different signalling protocols and transmission frequency bands.
Mobile telecommunication systems have been proposed that use satellite communication links between mobile user terminals and conventional terrestrial networks such as public switched telephone networks (PSTNs) and public land mobile networks (PLMNs). One network known as the IRIDIUM™ satellite cellular system is described in EP-A-0365885 and U.S. Pat. No. 5,394,561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.
Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed, with an orbital radius in the range of 10–20,000 km. Reference is directed to the ICO™ satellite cellular system described for example in GB-A-2 295 296. With this system, the satellite communications link does not permit communication between adjacent satellites. Instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to conventional land-based telephone networks. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM. Also simpler satellite communication techniques can be used than with a LEO network. Reference is directed to “New Satellites for Personal Communications”, Scientific American, April 1998, pp. 60–67, for an overview of LEO/MEO satellite networks.
Conventional GSM-based systems use a scheme based on a combination of time and frequency division multiple access to provide communication channels. The available bandwidth is divided into a number of carrier frequencies, each of which is further divided using a TDMA scheme. Speech and data are carried by a number of traffic channels (TCH).
To allow signalling messages to be conveyed along with the user data, each traffic channel is associated with a low rate channel, known as the Slow Association Control Channel (SACCH). This is used mainly for link maintenance and control procedures, such as transmission power control, timing advance control and the transmission of information relating to measurement reporting procedures to be performed by the user terminal.
The TDMA frame structure for TCH/SACCH channels according to the GSM Specifications is shown in FIG. 1. The basic unit of transmission is a series of about 100 modulated bits which is referred to as a burst. Bursts are sent in time and frequency windows which are referred to as slots. The duration of a slot is referred to as a burst period or BP and in the GSM system lasts 15/26 ms (approximately 0.577 ms). Eight burst periods are grouped into a TDMA frame, which lasts 120/26 ms (approximately 4.615 ms). Each traffic channel is based on one burst period per frame, so that eight TCH channels can be accommodated per frame. In a 26 frame multiframe, which lasts 120 ms, frames 0 to 11 and 13 to 24 each carry 8 channels of TCH data, while frame 12 carries SACCH data, each SACCH burst period providing the necessary signalling for one TCH channel. Frame 25 is unused. A complete SACCH message or block is distributed over four multiframes i.e. 480 ms so that 2 SACCH blocks are sent approximately every second.
Each time slot is associated with a unique number referred to as the Absolute Time Slot Number (ATN). For a 26 frame multiframe with 8 slots per frame, this runs from, for example, 0 to 207. The GSM specifications define the time slot number (TN) of a particular channel as the ATN mode 8, i.e. the remainder when the absolute time slot number is divided by the number of slots per frame. This is a number in the range 0 to 7 specific to a TCH channel. For example, ATN 22 (starting with slot number 0) lies in frames 3 and represents a TCH channel with TN=6.
The efficiency of a GSM system is increased by operating in discontinuous transmission mode (DTX), which reflects the fact that a user only speaks for a proportion of the time during normal conversation. During DTX operation, the traffic channel TCH is not seen. However, signalling is still required to maintain and control the link, so that the SACCH is still transmitted, together with frames known as Silence Descriptor (SID) frames. The purpose of SID frames is to transfer the characteristics of the background noise at the transmitter to the receiver. This feature aims to overcome the disturbance to the listener which has otherwise been shown to result from the sudden disappearance of background noise when the speaker stops speaking.
During DTX mode, to avoid an uneven load on the basis station transmitter, with all SACCHs being transmitted at frame 12, the emission time for the SACCH bursts can be spread evenly over the empty frames. This can be achieved by mutliplexing the SACCH burst according to the time slot number TN allocated to a particular channel. For example, for the frame structure described above, the SACCH burst can be transmitted on (ATN div 8=0) for TN=0, (ATN div 8=1) for TN=1, and so on, where a div b is the quotient when a is divided by b. Therefore, the SACCH burst associated with TCH channel 0 is emitted in frame 0, and that associated with channel 2 in frame 2, as shown in FIG. 2. Therefore, a single SACCH burst and a single SID burst are sent in each of frames 0 to 7 and nothing is emitted in any of the other frames. This scheme does not constrain the complete SACCH emission to take place during this frame number. For example, the SACCH may be multiplexed over, for example, eight frames beginning at a specified frame number.
In a satellite system, the satellite constellation can be configured so that for any location of user terminal on the earth, more than one satellite is at an elevation of more than 10 degrees above the horizon and hence two satellites are usually available for communication concurrently with the user terminal. The availability of more than one satellite permits so-called diversity operation which a traffic channel can be transmitted between the ground and the user terminal concurrently via two satellites, in two paths, to mitigate effects of blockage and fading. Diversity operation is described in GB-A-2 293 725 and EP-A-0 837 568.
In systems which support diversity operation, the separate physical paths for transmission make up a single logical path, so that the emissions on the diversity paths must correspond to identical encoded data. For dual path diversity, the user terminal is designed to receive two bursts per frame, for example a burst from a first satellite in time slot TN=0 and a burst from a second satellite at time slot TN=4. Since the time slot number TN will be different for the two diversity paths, multiplexing of the SACCH emissions cannot be based on the time slot number TN, since the SACCH data would not then arrive at the user terminal within the same frame. For example, using the TN based multiplexing scheme explained above, the SACCH burst from the second satellite would arrive at the user terminal 4 frames after the burst from the first satellite, so introducing significant delays in processing the SACCH signals.
In addition, a TN based multiplexing scheme can impair the ability of the system to use the time alignment of the SACCH frames as a unique reference for functions such as measurement reporting and burst alignment in DTC operation. It can also facilitate attacks on the encryption scheme in systems where data is different frames is encrypted using different encryption keys.